Negative synchronous wave amplifier and oscillator



Jan. 3, 1967 R. ADLER 3,296,548

NEGATIVE SYNCHRONOUS WAVE AMPLIFIER AND OSCILLATOR 5 Sheets-Sheet 2Original Filed June 2'7, 1961 \illie I 21111:: 0 2 ffg.

Jan. 3, 1967 R. ADLER 3,296,548

NEGATIVE SYNCHRONOUS WAVE AMPLIFIER AND OSCILLATOR Original Filed June27, 1961 3 Sheets-Sheet 5 Ffa. 7

INVENTOR.

7R0 be??? QQUZZeZ BY fiiii Patented Jan. 3, 1967 3,296,548 NEGATIVESYNCHRONOUS WAVE AMPLIFIER AND OSCILLATOR Robert Adler, Northfield,Ill., assignor to Zenith Radio Corporation, Chicago, 111., a corporationof Delaware Original application Jan. 27, 1961, Ser. No. 119,931.Divided and this application July 13, 1966, Ser. No. 564,858

5 Claims. (Cl. 330-46) This application is a division of copendingapplication Serial No. 119,931 filed June 27, 1961, by Robert Adler andassigned to the same assignee.

The present invention pertains to electron beam amplifiers and apparatustherefor. It has particular reference to electron couplers andcombinations therewith.

As disclosed in the copending application of Robert Adler, Serial No.738,546, filed May 28, 1958, for Electronic Signal Amplifying Apparatusand Methods, now Patent No. 3,233,182, and assigned to the same assigneeas the present application, of which the aforesaid parent application isa continuation-in-part, it is known that interaction between electronbeams and circuits placed alongside such beams can take different forms.Tha application specifically describes apparatus which interacts withand amplifies either of two distinct electron waves, the fast and slowelectron waves which have velocities respectively faster and slower thanthe average electron beam velocity along the beam path. Electron waveaction may be considered as characteristic of an electron beam which issubjected to a restoring force tending to establish a resonant orelastic suspension for the beam electrons. In transverse-field tubes,the restoring force enables each electron in the beam to oscillate aboutits rest position at a frequency known as the transverse or cyclotronresonance frequency. Motion of the electrons in the beam at the electronresonant frequency, once excited, persists until disturbed by some othermechanism such as an amplifying section or an output section.

Electron motion may be excited by a helix or equivalent circuit whichhas a velocity of wave propagation properly selected so that theelectrons are subjected to a signal field at the electron resonancefrequency. The conventional traveling wave tube exemplifies that type ofdevice in which the interaction process develops slow waves. On theother hand, certain electron couplers described in the aforesaidcopending application are characterized by interaction with theelectrons to develop fast wave signal energy on the electron beam. Onesuch coupler is of the lumped-electrode type which is characterized byits property of infinite phase velocity. This device exhibits maximuminteraction with the beam when the applied signal frequency equals theelectron resonance frequency. However, for certain applications, acoupler is required which optionally interacts at signal frequenciesdifferent from the electron resonance frequency.

In addition to fast and slow cyclotron waves, signal energy may becarried on an electron beam by the synchronous waves. The detailedcharacteristics of synchronous waves are described in an articleentitled Waves on a Filamentary Electron Beam in a Transverse-FieldSlow-Wave Circuit by A. E. Siegman and appearing in the Journal ofApplied Physics, Volume 31, No. 1, pages 1726 for January 1960.Synchronous wave energy appears in two different forms, thepositive-energy carrying wave and the negative-energy carrying wave; forsimplicity, these two waves will be referred to hereinafter simply asthe positive and negative synchronous waves. Since both of these waveshave the same phase velocity, it is not possible to distinguish betweenthem in the manner that couplers distinguish between fast and slowcyclotron waves.

It is a general object of the present invention to provide a new andimproved electron coupler having utility in one or more of theapplications discussed above.

Another object of the present invention is to provide a new and improvedelectron coupler capable of efl'iciently exciting resonant electronsignal motion for signal energy at a frequency other than that ofelectron resonance.

A further object of the present invention is to provide a new andimproved electron coupler capable of selectively interacting with anddistinguishing between synchronous waves.

A related object of the present invention is to provide new and improvedapparatus for amplifying synchronous wave energy.

In one aspect of the present invention, an electron coupler is designedfor interaction with a synchronous wave. The coupler includes a bifilarhelix which is coaxial with the electron beam path and has a pitch suchthat where n is the number of helix turns per unit length, n is thenumber of electron cyclotron orbits per unit length and f is theelectron cyclotron frequency, the average electron velocity being equalto f/n and establishing a condition of synchronous-wave operation in allactive sections of said device.

In a synchronous wave amplifier constructed in accordance with theinvention, the bifilar helix electron coupler is arranged to subject theelectron to interacting transverse field forces which travelsynchronously with the electrons of the beam. When the synchronous wavethus developed is of the negative type, amplification of the synchronouswave energy is achieved by coaction of the synchronous wave with thecoupler.

The features of the present invention which are believed to be novel areset forth with particularity inthe appending claims. The organizationand manner of operation of the invention, together with further objectsand advantages thereof, may best be understood by reference to thefollowing description taken in connection with accompanying drawings, inthe several figures with which like reference numerals identify likeelements and in which:

FIGURE 1 is an elevational view, partially broken away, of oneembodiment of the present invention;

FIGURES 1a, 1b, and 1c are cross-sectional views taken along the lines1a1a, 1b1b, and 1c-1c, respectively, in FIGURE 1;

FIGURE 2 is an enlarged fragmentary view of a portion of the apparatusshown in FIGURE 1;

FIGURE 2a is an enlarged fragmentary cross-sectional view taken alongthe lines 2a2a in FIGURE 2; and

FIGURES 3-11 are schematic diagrams each depicting a differentembodiment of the invention claimed herein, in the parent hereof or inanother division of the latter.

Referring to FIGURE 1 which serves to illustrate the present inventionas will become apparent later, enclosed within an evacuated envelope 10are an electron gun 11, first and second input couplers 12 and 13disposed in a first beam path portion beyond gun 11, a parametricelectron motion expander 14 disposed in a second beam path portionbeyond the first, an output coupler 15 disposed in a third path portionbeyond expander 14,- and a collector 16. In operation, the entireelectron beam path is subjected to a longitudinal magnetic fieldindicated by arrow II. It is usually most convenient to develop themagnetic field by means of a solenoid (not shown) within which envelope10 is coaxially disposed.

Electron gun 11 is designed to develop and project along the beam path apencil-like stream of electrons at least approximating a condition ofBrillouin flow. To this q is end, the gun includes a cathode 18 followedby a series of annular electrodes 19. The aperture in the firstelectrode beyond the cathode is small so as to accept only a centralportion of the electron stream emitted from cathode 18. The annularelectrodes are spaced and energized so as to first confinethe beamessentially to a cross-over, simulating a point source, and then permitthe beam to expand and attain that diameter at which the divergent spacecharge forces are balanced by the confining force of the magnetic field.The electrons projected along the beam path from gun 11 are finallyintercepted by collector 16 after passing suppressor electrode 20. Theaverage axial velocity of the electrons is adjusted by the potentialapplied to the elements in each of the intermediate sections 12-15.

Input section 12 is an electron coupler which interacts with the beamelectrons at a desired interaction signal frequency 1. Two conductivestrips 22, 23 (FIGURES la, 2 and 2a) are interleaved and wound coaxiallyabout the beam path to form a bifilar helix coaxial with the path. Strip22 is electrically and mechanically connected at each of its ends to asupport 24 disposed along one side of the beam path. Strip 23 issimilarly connected at each of its ends to a support 25 on the oppositeside of the beam path. Supports 24 and 25 are mounted between insulatingspacers 26. The coupler is tuned to its interaction frequency by aninductive loop 30 connected across supports 24, 25. Suitable conductiveleads 27 connect opposite sides of loop 30 with appropriate base pins 28extending through one end of envelope 10.

The frequency at which the coupler interacts with the beam electrons isa function of its pitch and direction of twist relative to theperiodicity and direction of the resonant electron motion. For thegeneral case of transverse resonant motion of the electrons, the pitchof the coupler. must correspond with that of the electron wave pattern.The number of n of resonant motion periods per unit length is expressedby the equation where u is the average electron velocity in the axialdirection and f, is the frequency of electron resonance. To interactproperly with electron waves, the pitch of the bifilar helix couplermust be such that where n is the number of helix turns per unit lengthand f is the desired interaction frequency. 1

More specifically with respect to the device illustrated in FIGURE 1,the application of signal energy to the electron beam causes theelectrons to follow helical orbits having a periodicity determined bythe strength of magnetic field H in accordance with the well understoodcyclotron relationship; the cyclotron frequency i in megacycles is equalto 2.8H, where H is the strength of the field in gauss. From Equation 1,the number 12 of cyclotron orbits per unit length is expressed bySimilarly from Equation 2, the pitch of the bifilar helix must be suchthat By constructing coupler 12 in accordance with Equation 4, itsinteraction frequency is optimized at a value which is different fromthe cyclotron frequency. In the specific embodiment shown, coupler 12has a twist opposite that of the orbital electron paths in order to beoperative at an interaction frequency higher than the cyclotronfrequency. This is in accordance with Equation 4 which gives a negativesign to n for f f Defining the direction of field H indicated in FIGURE1 as that which pro- 4. duces counter-clockwise clockwise orbitaltrajectories as viewed from the cathode, coupler 12 in this instance istwisted in the clockwise direction.

To interact properly with the beam, the bifilar strips should develop ahomogeneous field in the region traversed by the electrons. As shown inFIGURE 20, this condition is approximated by curving the striptransverse cross-section to present an inwardly facing concave surface.Each of the strip transverse cross-sections may extend about 90 aroundthe beam path and the radius of the coupler preferably is such that itspitch is larger than its circumference.

Electron coupler 13 in the instant embodiment is structurally similar toelectron coupler 12. However, its assigned interaction frequency islower than the cyclotron frequency so that it is twisted in the oppositedirection from coupler 12, i.e., in the same direction as the cyclotronorbits, as indicated by the resultant sign of Equation 4. It includestwo strips 32, 33 (FIGURE 1b) respectively on supports 34, 35 securedbetween micaspacers 36, 37. An inductance coil 38 connected acrosssupports 34, 35 tunes coupler 13 to its interaction frequency.Connecting leads 39 are tapped across coil 38 and extend through the endof envelope 10 by way of appropriate ones of base pins 28.

Output section 15 is essentially identical in this instance to inputsection 13. It includes a bifilar helix composed of strips 40 and 41twisted in the same direction and with the same pitch strips 32, 33. Itis tuned to its interaction frequency by an inductane coil 45 bridgedacross supports 46, 47 secured between spacers 48, 49. Leads 50 extendfrom appropriate impedance-matching taps on coil 45 through the otherend of envelope 10 by way of base pins 50.

Expander section 14 may take the form of any device appropriate toparametrically amplify the signal energy imposed upon the beam by one ofthe input signal sections. In the preferred embodiment illustrated inFIG- URE l, amplification is achieved by means of a quadrupoleparametric expander. This particular expander is described and claimedin the co-pending application of Glen Wade, Serial No. 747,764, filedJune 10, 1958, entitled Parametric Amplifier, and assigned to the sameassignee as the present invention. It has also been described in anarticle entitled a Low-Noise Electron-Beam Parametric Amplifier by Adleret al. which appeared in the Proceedings of the IRE, Volume 46, No. 10,for October 1958. The expander includes four electrodes 52 spacedcircumferentially around the beam path and supported between insulatingspacers 53, 54. Connected between each adjacent pair of electrodes 52 isan inductive loop 55, the loops together tuning the structure to thefrequency f of the pump signal to be applied from an external pumpsource. The latter signal is inductively coupled to one of loops 55 by afeed loop 56 fed with the pump signal energy by suitable leads 57extending through the end of envelope 10 by way of appropriate ones ofbase pins 50. Oppositely facing ones of electrodes 52 are strappedtogether to insure operation in the pi mode.

The entire assembly is rigidly supported by means of insulator rods 58which pass through openings in the different annular electrodes andinsulating spacers supporting the individual sections and related parts.Insulating sleeves or washers 59 disposed over rods 58 space may bereduced by the use of inductive or capacitive coupling directly to eachof the sections through envelope 10, instead of by means of theconnecting leads and base pins.

In operation, input coupler 13 and output coupler are constructed tointeract with the electron beam at the frequency i of the input signalto be amplified in expander section 14. When coupler 13 is fed by energyfrom the external input signal source, the electrons passing through thecoupler describe expanding helical orbits, the periodicity of which isthat of the established cyclotron resonance frequency and the radius ofwhich is proportional to the strength of the input signal.

Upon subsequently entering expander section 14, the electrons aresubjected to a periodic inhomogeneous quadrupole field which impartsenergy to the electron signal motion and thereby amplifies the signalintelligence. The amplified signal intelligence is derived from the beamin output coupler 15 and fed to an external load coupled thereto. Theorbiting electrons give up energy to the bifilar helix of coupler 15 ina manner reciprocal to the action of input coupler 13.

The pump signal frequency f,, is approximately twice the cyclotronfrequency f As a result of the parametric amplification process, theidler signal f is developed at a frequency which is in this case higherthan the cyclotron frequency. Noise energy on the electron beam at theidler frequency is capable of being converted, during the parametricamplification process, to the input signal frequency as a result ofwhich it appears in the output signal. To minimize this, the idlerfrequency noise cornponents are stripped from the beam prior to theamplification process. The idler stripping is achieved by coupler 12which interacts at the higher idler frequency. Idler signal coupler 12preferably is coupled to and terminated in an external noise sinkproperly matched thereto.

To illustrate the operational relationships of the different sections,and in no sense by way of limitation upon the structure, it may beassumed that the magnetic field strength is of a value establishingcyclotron resonance at 1,000 megacycles. The frequency of the pumpsignal applied to expander 14 is 2,000 megacycles. The bifilar helix ofcoupler 13 is wound in the same direction as the direction of cyclotronmotion and its pitch is selected in accordance with Equation 4 so thatits interaction frequency is 400 megacycles. Because the idler signaldeveloped by the parametric process has a frequency equal to thedifference between the input and pump signal frequencies, electroncoupler 12 is constructed to interact at 1600 megacycles. Its pitch alsois determined in accordance with Equation 4 and its direction of twistis opposite that of the electron cyclotron motion. The 400 megacycleinput signal energy is modulated upon the beam in coupler 13, amplifiedin expander 14, and extracted by output coupler 15. Noise energycomponents at the 1600 megacycle idler frequency are stripped from thebeam by coupler 12, in this case prior to entry of the beam into inputcoupler 13, and therefore are removed prior to the parametricamplification process which would convert a portion of the noise energyto the input signal frequency at which the output coupler interacts.

Numerous combinations of the described bifilar helix couplers withdifferent kinds of input, expander, and output sections permit variedoperational results. In all cases, amplifying section 14 is constructedto subject the electrons to a pump signal field which has a wave numberequal to the algebraic sum of the signal and idler wave numbers, thewave number being the number of waves per unit length. In the input andoutput sections the wave number is equal to the number of turns per unitlength, previously defined as n, with the same positive or negativesign. The illustrated signal expander exhibits an infinite phasevelocity for the pump signal wave, while the couplers themselves exhibita finite phase velocity in the conventional sense; however, it isimportant to note that the action of the above described couplers isthat of a lumped-electrode device, as opposed to the distributed actionnormally associated with waves traveling at a finite velocity.Lumped-electrode action is assured by connecting both ends of each helixelement to the same support element. Even without the common endconnections, lumped action occurs so long as the electrical length ofthe conduction elements or strips 22, 23 etc. does not exceed one-halffree-space wavelength at the frequency at which they operate. Because ona lumped element phase is the same everywhere at a given instant, thespacing and the twist of the turns corresponds to the spacing and twistof the cyclotron wave with which the coupler interacts.

Another embodiment is illustrated schematically in FIG- URE 3 in whichthe characteristics of the different sections are again selected so asto separate the idler and signal frequencies. In this instance, theprincipal sections of the device include an infinite phase velocityinput signal coupler 65, a bifilar helix coupler 66 constructed tointeract at the idler frequency, a quadrupole parametric expander 67 thefour electrodes of which are skewed around the beam path, and aninfinite phase velocity output coupler 68.

The input and output couplers 65, 68 are constructed of a simpler pairof deflector plates disposed on opposite sides of the beam path. Thelumped capacitance of the deflection plates is tuned to the interactionfrequency by a suitable matching network between the coupler and theexternal signal source or load. The action of coupler 65, 68 is fullydescribed in the aforementioned Adler application Serial No. 738,546 andalso is discussed in detail in an article by Adler et al. entitledParametic Amplification of the Fast Electron Wave which appeared in theProceedings of the IRE for June 1958, volume 46, No. 6. The interactionfrequency of couplers 65, 68 is optimum at the cyclotron frequencyestablished by magnetic field H.

Quadrupole 67 is skewed in the manner disclosed and claimed in theaforementioned Wade application so as to properly respond to a pumpsignal having a frequency different from twice the cyclotron frequency.The amount by which the quadrupole electrodes are twisted about the beampath is such that by Doppler effect the orbiting electrodes see aquadrupole pump field alternating at twice the cyclotron frequency. Thisis compatible with the condition that the wave number of the pump fieldmust be equal to the sum of the idler and input signal wave numbers.

Idler noise stripper 66 is assigned a pitch such that its number ofturns per unit length corresponds to the idler wave number. Assuming,for example, that it is desired to separate the idler and signalfrequencies so that the idler frequency is 1.3 times the input signalfrequency, from Equation 4 the number of turns per unit length of theidler signal coupler 66 is .3 times the number of cyclotron orbits perunit length. Since the idler signal frequency is higher than thecyclotron frequency, the direction of twist of coupler 66 is oppositethat of the electron orbital motion. In this specific example, thesignal wave number is zero and thus idler noise stripper 66 andquadrupole 67 have the same wave number. The amount by which thequadrupole electrodes are skewed therefore is such that the pumpfrequency is 2.3 times the cyclotron frequency.

It has been found that the interaction effects of a finite phasevelocity coupler tend to deteriorate gradually as the electron beammoves on beyond such a coupler. For this reason, the idler coupler ofFIGURE 3 is disposed in the initial beam path portion beyond inputsignal coupler 65 in order to perform its function just prior to theparametric amplification process effected by quadrupole expander 67. Ingeneral, however, the order of arrangement of the two electron couplersalong the initial beam path portion may be reversed.

Another embodiment of the present invention as shown in FIGURE 4 employsinput and output couplers 70, 71 constructed to interact with inputsignal waves of finite phase velocity. An idler coupler 72 interacts atan infinite phase velocity, and a traveling-wave type parametricexpander 73 serves to amplify the input signal waves. Expander 73 iscomposed of four helices 74 spaced circumferentially around the beampath and individually oriented parallel thereto. In a geometrical sense,the device of FIGURE 4 is somewhat the converse of that illustrated inFIGURE 3. Idler coupler 72 interacts with the electron beam at thecyclotron frequency. Separation between the idler and signal frequenciesis obtained by the use of bifilar helix couplers 70 and 71 to which apitch is assigned such that they interact at a desired input signalfrequency lower than the cyclotron frequency. In this instance, theidler wave number is zero and the wave numbers of signal couplers 70, 71and expander 73 are the same. Accordingly, the pump signal applied toparametric expander 73 has a frequency less than twice that of thecyclotron resonance condition. Because of the lower signal frequency,the signal wave travels toward the cathode and the pump signal thereforeis fed to the end of helices 74 nearest the collector. For clarity inthis and subsequent figures the actual connections to the varioussections, as well as the cathode and collector, have been omitted in thedrawings; except as otherwise noted herein, the arrangements are thesame as depicted in FIG- URE 3, with the cathode on the left.

Still another embodiment of the present invention is shown in FIGURE 5.All four sections of the device are constructed for waves having afinite phase velocity. Quadrupole expander 75 is skewed in the manner ofexpander 6'7 discussed in connection with FIGURE 3 so as to respond to afrequency different from twice the cyclotron frequency established bymangetic field H. Input and output electron couplers 76 and 77,respectively, are assigned a pitch in accordance with Equation 4 toestablish an interaction frequency less than the cyclotron frequency.The bifilar helix of electron coupler 78 is assigned a pitch forinteraction at an idler frequency higher than the cyclotron frequency.Because of the invariant relationship which must exist between the wavenumbers of the three different signal frequencies involved, the wavenumber (or correspondingly the number of turns per unit length) of idlercoupler 78 is in this embodiment separated from the cyclotron frequencyby an amount different from the amount by which the input signalfrequency departs from the cyclotron frequency; accordingly, its pitchis tighter than that of the other couplers.

Considering the embodiments of FIGURES 15 in retrospect, it will beapparent that the use of different combinations of finite and infinitephase velocity sections enables separation of idler and signalfrequencies while yet meeting a variety of different signalspecifications. By assigning a number of turns per unit length inaccordance with Equation 4, the desired signal frequency may be causedto interact properly at the cyclotron frequency of the device. Thedirection in which the interaction frequency departs from the cyclotronfrequency is determined by the relative direction of twist of the helixas compared with that of the cyclotron orbits.

The embodiments discussed thus far have each included a gain oramplifying section. When this section is not used, or is omittedentirely from the tube, the resultant device retains utility in serviceas an isolator because it will translate signal energy only in thedirection of electron travel. Alternatively, a mixer or amplitudemodulator section may be interposed along the beam path between inputand output couplers. Still further, the idler stripping coupler may beomitted when the tube is to be utilized in applications where the noiseof the idler channel may be disregarded.

As described above, the electron couplers of the invention areconstructed to interact with the fast electron wave. Interaction withslow electron waves likewise is achievable with the bifilar helixcouplers. To this end the coupler is constructed in accordance withEquation 2 or 4 with a negative sign accorded to the signal frequencyterm 1. Hence. the pitch of the coupler always is positive, designatinga coupler twist in the same direction as that of the cyclotron orbits.For a given electron velocity u, the number of turns it will be largerfor interaction with a slow wave than with a fast wave. On the otherhand, any of the couplers previously described interact with the slowwave upon an appropriate increase in the electron velocity u. As thusfar described, the invention is claimed in the parent applicationhereof.

The present invention also pertains to apparatus in which the signalenergy is carried by the synchronous electron wave. As explained in theaforementioned article by Siegman, synchronous waves differ fromcyclotron waves in several respects. Two different synchronous waves mayexist on the beam, one of which is defined as a positive energy-carryingwave and the other of which is a negative energy-carrying wave. Forconvenience, these two waves will hereinafter be referred to simply asthe positive and the negative synchronous waves. In some respects, thesetwo waves appear to correspond with the slow and fast cyclotron waves.Couplers for interacting with cyclotron waves are capable ofdistinguishing between the slow and fast waves by virtue of theirdifferent phase velocities. However, .separation on the basis of phasevelocity is not possible for synchronous wave interaction because thepositive and negative synchronous waves have the same phase velocity. Toselectively interact with but one of the synchronous waves, an electroncoupler must be constructed to develop fields in two coordinatedirections transverse to the beam path. The bifilar helix electroncoupler structures of the present invention are uniquely suited to thisrequirement. A simple synchronous wave device is illustrated in FIG- URE6 which, as in FIGURES 4 and 5, diagrammatically illustrates only theessential elements of the device while not depicting the cathode,collector, source and load. Its basic sections are an input coupler andan output coupler 81. Each of these couplers is physically constructedin the same manner as coupler 12 described with respect to FIGURE 1.

The electron pattern of a positive synchronous wave twists in thedirection of the electron orbits. The couplers in the device of FIGURE 6are specifically intended for interaction with the positive synchronouswave and therefore have a twist in the same direction as the orbitalelectron motion caused by magnetic field H. The pitch of couplers 80,81, which must be the same as that of the synchronous wave electronpattern to interact therewith, is selected in accordance with theequation r 1L where n is the number of helix turns per unit length, 1 isthe interaction frequency, and u is the average axial electron velocity.Substituting from Equation 3, Equa tion 5 becomes So constructed, theelectron velocity is such that a given electron passes one complete turnof the helix for each signal cycle. The assumed electron effectively isunder the influence of a DC. field which causes the elec' tron to driftat right angles to that field as well as to the axial magnetic field.Because of the circular symmetry of the bifilar helix, correspondingforces are exerted on all electrons and the direction in which any givenelectron moves depends on its phase at entry into the coupler section.Consequently, the pencil beam is spread into a corkscrew of growingdiameter. The corkscrew pattern has the same direction of twist as thebifilar windings and in passing through the coupler structure theelectron pattern induces current in the coupler at the signal frequency.In the present example, the phase 9 of this current is such as toconstitute a positive conductance load on the coupler. Thus, the deviceshown on FIGURE 6 constitutes a unidirectional signal energy translatorin which the energy is carried on the electron beam by the positivesynchronous wave.

In practice, the device of FIGURE 1 may be operated in the mannerdescribed with respect to FIGURE 6, merely by utilizing input and outputcouplers 13 and 15, disabling idler stripper 12 and expander 14, andproperly adjusting the electron beam accelerating voltage so as tosatisfy Equation 6. For example, with couplers 13 and 15 constructed asdescribed for cyclotron wave interaction at 400 megacycles, lowering thebeam voltage a few volts (in an exemplary tube from about 7 to 4 volts,true voltage) permits positive synchronous wave transmission.

By reversing either the direction of twist of couplers 80, 81 in FIGURE6 or the direction of magnetic field H, interaction with the negativesynchronous wave is obtained since the twist of its electron pattern isopposite that of the positive synchronous wave. The later alternative isillustrated in FIGURE 7 in which couplers 80, 81 are the same as inFIGURE 6 but the direction of the magnetic field is reversed. Similarly,the device shown in FIGURE 1 may be caused to operate with a negativesynchronous wave by reversing the current in the solenoid encircling thetube. In this mode of operation, the beam constitutes a negativeconductance load upon the electron coupler. While there is no gain fromcoupler 80 to coupler 81, amplification is observed in this devicebecause of the effective negative conductance in each of the couplers.In order to prevent oscillation, it is necessary to carefully load thetwo couplers respectively by the signal source and by the utilizationdevice.

To amplify the positive synchronous wave, a parametric expander may beinserted between couplers 80, 81. Illustrative combinations are shown inFIGURES 8, 9 and 10. In FIGURE 8, a D0. energized quadrupole parametricexpander 84 is employed between couplers 80, 81. Quadrupole 84 isconstructed in the manner of expander section 14 of the deviceillustrated in FIGURE 1. In this case, the two oppositely facing pairsof expander electrodes are connected across a DC potential source. Insynchronous wave operation, the corkscrew pattern in which the electronsare arranged moves forward as a whole with the stream, while theindividual electrons maintain their positions with reference to the beamaxis. There is no transverse motion of the individual electronsinvolved. When the group of electrons arranged according to such apattern enters the D.C. quadrupole field, which for simplicity may bethought to be concentric with the pattern axis, each individual electronfinds itself in a DC. field the strength of which is proportional to thespacing of the electron from the axis and the direction of which isrelated to the azimuth of the specific electron. There are fourazimuthal positions where the DC. field is purely tangential. Because inthe magnetic field electrons move at right angles to the applied D.C.field, the electrons in the four positions just mentioned will moveradially, two inward and two outward. As they so move, they driftradially into regions of weaker and stronger tangential field,respectively. Therefore, their transverse drift must be exponential withrespect to time or axial distance.

At the end of the quadrupole, the original corkscrew pattern appearsstrongly distorted and flattened: it has grown exponentially along onetransverse axis and has been squeezed together, also exponentially,along the other. The resulting pattern is almost fiat; it can be thoughtof as consisting of two synchronous Waves, one positive and one negativewith both having the original signal frequency and being much larger inamplitude than the original pattern.

To summarize, the DC. energized quadrupole amplifies one transversecomponent of the positive synchro- 10 nous Wave and suppresses theorthogonal transverse component. Noise components of the negativesynchronous wave also are amplified.

The device shown in FIGURE 9 utilizes a quadrupole parametric expander86 in which each of the four quadrupole electrodes is a simple helix.Any pump signal frequency may be selected by assigning a propagationconstant to the helices such that the pump wave has a propagationvelocity equal to the average axial electron velocity u. The pump signalis fed to the end of the helices nearest the cathode (to the left inFIGURE 9). The amplification mechanism is equivalent to that explainedin connection with FIGURE 8; the individual electrons appear to see aDC. field.

The device shown in FIGURE 9 also amplifies a second or idlersynchronous wave at a frequency equal to the difference between the pumpand signal frequencies. When this difference is positive the idler waveis a positive synchronous wave, permitting removal of beam noise bymeans of a bifilar helix coupler constructed in accordance with thisinvention. Pump structure 86 does not discriminate between positivelyand negatively rotating components of the pump field. For this reason,the same structure also amplifies negative synchronous waves, among thema wave at the signal frequency and another wave at the idler frequency.Beam noise of these negative energy-carrying waves cannot be removed, sothat this device is not capableof amplification with extremely lowno1se.

FIGURE 10 shows a modification of the quadrupole pump structure in theform of four helices 88 spaced circumferentially around and twistedabout the beam axis. When a pump signal is applied to the left orcathode end of this structure, a pump wave is generated thecounterrotating components of which have unequal angular velocities. Therotation resulting from the geometrical twist is added to one componentbut subtracted from the other. This device therefore pumps with a waveadapted to amplifying positive synchronous waves, with the correct axialvelocity, while not similarly amplifying negative synchronous waves.Accordingly, low noise amplification is obtained when the pump frequencyis higher than the signal frequency and the beam noise at the resultingpositive idler Wave is removed by a bifilar helix coupler of theinvention.

An interesting synchronous wave device is illustrated in FIGURE 11. Itincludes but a single bifilar helix coupler 90. The helix is wound in adirection opposite the direction of cyclotron rotation defined bymagnetic field I-I. As described above with respect to FIGURE 1, thesynchronous wave interaction process produces a negative conductance incoupler 90 which is connected across a load 91. Adjustment of the loadto present a resistance larger than the amount of negative resistanceimpressed thereon by the electron beam causes the system to oscillate atthe synchronous wave interaction frequency. Energy to sustain theoscillations is derived from the direct current field which acceleratesthe beam. Similar results are obtained by constructing coupler 90 tointeract with the slow electron wave, as previously described.

The principles upon which the present invention are based are applicableto the development of a number of other synchronous wave devices. Fortraveling-wave interaction, the electron coupler must exhibit a phasevelocity equal to the speed of electron travel, the apparent phaserotation of a given cross-section of the electron beam must have aclearly defined direction, such as the direction of electron motion in amagnetic field, and that same direction must be defined by the externalcircuit. Any such device must involve interaction in two coordinatetransverse dimensions. For example, a synchronous traveling wave tubeutilizing these principles may take the form of a helix coiled about thebeam axis with a pitch such that the number of wavelengths on one majorturn equals the number of synchronous wave lengths plus or minus one(depending on the direction of orbital electron rotation) on acorresponding length of the beam. This specific subject matter isdescribed and claimed in the co-pending application of Robert Adlerentitled Election Beam Devices, Serial No. 205,591, filed June 27, 1962,now Patent No. 3,218,503, and assigned to the same assignee as thepresent invention.

The numerous different embodiments disclosed illustrate the versatileapplicability of a bifilar helix constructed to interact with theelectron beam in the manner of a lumped-electrode coupler. As part of afast wave amplifier, the electron coupler of the invention permitsseparation of signal and idler frequencies. It also serves to increasethe flexibility of choice of the various different signal frequenciesinvolved. The present application also describes several differentsynchronous wave devices. The bifilar helix electron coupler isespecially suited for interaction with the synchronous wave because itsatisfies the requirement of interaction in two transverse coordinatedirections so that it may selectively interact with only one of the twosynchronous waves. Claims specific to the positive synchronous wave modeof operation described herein, and also claims dominant to the inventionclaimed herein, appear in copending application Serial No. 564,856 filedJuly 13, 1966, which also is a division of the aforesaid applicationSerial No. 119,931.

While particular embodiments of the present invention have been shownand described, it will be obvious to those skilled in the art thatchanges and modifications may be made without departing from theinvention in its broader aspects. Accordingly, the aim in the appendingclaims is to cover all. such changes and modifications as fall withinthe true spirit and scope of the invention.

I claim:

1. A synchronous-wave signal energy translator and amplifier comprising:

means for projecting an electron beam along a pre determined path;

means for establishing cyclotron resonance for electrons in said beam;

a first electron coupler responsive to input signal energy fordeveloping a field, having components in two coordinate directionstransversely across said path and propagating along said pathsynchronously with said electrons, which imparts to said beam anelectron pattern defining a negative synchronous wave and having a twistabout said path in a direction opposite that in which said electronsorbit in correspondence with said cyclotron resonance;

and a second electron coupler, spaced along said path from said firstcoupler and coupled to a signal-energy load, for establishing a fieldhaving components in two coordinate directions transversely across saidpath and propagating along said path synchronously with said electronsto interact with the negative synchronous-wave defined by said electronpattern.

2. In an electron beam device of the synchronous-wave type, an electroncoupler for interacting with the negative synchronous electron wavecomprising:

a lumped-electrode bifilar helix coaxial with the electron beam path andhaving a twist of a sense opposite that of the electron cyclotron orbitsand a pitch such that Where n is the number of helix turns per unitlength, n

is the sumber of cyclotron orbits per unit length, f is the desiredinteraction frequency, and f is the cyclotron frequency.

3. A device as defined in claim 2 which includes a second bifilar helixcoaxial with and disposed downstream from the first and having a twistof a sense opposite that of the electron cyclotron orbits and a pitchsatisfying the aforesaid equation.

4. A synchronous-wave signal energy oscillator comprising:

means for projecting an electron beam along a predetermined path;

means for establishing cyclotron resonance for electrons in said beam;

means disposed along said path for interacting with said beam to developsynchronous-wave energy thereon selectively in only the negative energycarrying mode; and a load coupled to said interacting means andpresenting a resistance thereto greater in value than the resistancevalue presented to said interacting means by said beam.

5. An oscillator as defined in claim 4 wherein said interacting means isa lumped-electrode bifilar helix coaxial with the electron beam path andhaving a twist of a sense opposite that of the electron cyclotron orbitsand a pitch such that where n is the number of helix turns per unitlength, 21 is the number of cyclotron orbits per unit length, f is thedesired interaction frequency, and f is the cyclotron frequency;

said load being coupled across the windings of said helix.

No references cited.

ROY LAKE, Primary Examiner.

D. HOSTETTER, Assistant Examiner.

1. A SYNCHRONOUS-WAVE SIGNAL ENERGY TRANSLATOR AND AMPLIFIER COMPRISING:MEANS FOR PROJECTING AN ELECTRON BEAM ALONG A PREDETERMINED PATH; MEANSFOR ESTABLISHING CYCLOTRON RESONANCE FOR ELECTRONS IN SAID BEAM; A FIRSTELECTRON COUPLER RESPONSIVE TO INPUT SIGNAL ENERGY FOR DEVELOPING AFIELD, HAVING COMPONENTS IN TWO COORDINATE DIRECTIONS TRANSVERSELYACROSS SAID PATH AND PROPAGATING ALONG SAID PATH SYNCHRONOUSLY WITH SAIDELECTRONS, WHICH IMPARTS TO SAID BEAM AN ELECTRON PATTERN DEFINING ANEGATIVE SYNCHRONOUS WAVE AND HAVING A TWIST ABOUT SAID PATH IN ADIRECTION OPPOSITE THAT IN WHICH SAID ELECTRONS ORBIT IN CORRESPONDENCEWITH SAID CYCLOTRON RESONANCE; AND A SECOND ELECTRON COUPLER, SPACEDALONG SAID PATH FROM SAID FIRST COUPLER AND COUPLED TO A SIGNAL-ENERGYLOAD, FOR ESTABLISHING A FIELD HAVING COMPONENTS IN TWO COORDINATEDIRECTIONS TRANSVERSELY ACROSS SAID PATH AND PROPAGATING ALONG SAID PATHSYNCHRONOUSLY WITH SAID ELECTRONS TO INTERACT WITH THE NEGATIVESYNCHRONOUS-WAVE DEFINED BY SAID ELECTRON PATTERN.